Conventionally, as display elements included in a display device, there are an electro-optical element whose luminance is controlled by a voltage applied thereto, and an electro-optical element whose luminance is controlled by a current flowing therethrough. A representative example of the electro-optical element whose luminance is controlled by a voltage applied thereto includes a liquid crystal display element. On the other hand, a representative example of the electro-optical element whose luminance is controlled by a current flowing therethrough includes an organic EL (Electro Luminescence) element. The organic EL element is also called an OLED (Organic Light-Emitting Diode). An organic EL display device using organic EL elements which are self light-emitting type electro-optical elements can easily achieve slimming down, a reduction in power consumption, an increase in luminance, etc., compared to a liquid crystal display device that requires a backlight, color filters, and the like. Therefore, in recent years, there has been active development of organic EL display devices.
As the driving system of an organic EL display device, there are known a passive matrix system (also called a simple matrix system) and an active matrix system. An organic EL display device adopting the passive matrix system is simple in structure, but is difficult to achieve size increase and definition improvement. On the other hand, an organic EL display device adopting the active matrix system (hereinafter, referred to as “active matrix-type organic EL display device”) can easily achieve size increase and definition improvement, compared to the organic EL display device adopting the passive matrix system.
The active matrix-type organic EL display device has a plurality of pixel circuits formed in a matrix form. Each pixel circuit of the active matrix-type organic EL display device typically includes an input transistor that selects a pixel, and a drive transistor that controls the supply of a current to an organic EL element. Note that in the following the current flowing through the organic EL element from the drive transistor may be referred to as “drive current”.
Meanwhile, in a general active matrix-type organic EL display device, one pixel is composed of three subpixels (an R subpixel that displays red, a G subpixel that displays green, and a B subpixel that displays blue). FIG. 37 is a circuit diagram showing a configuration of a conventional general pixel circuit 91 forming one subpixel. The pixel circuit 91 is provided corresponding to each of intersections of a plurality of data lines DL and a plurality of scanning signal lines SL which are disposed in a display unit. As shown in FIG. 37, the pixel circuit 91 includes two transistors T1 and T2, one capacitor Cst, and one organic EL element OLED. The transistor T1 is a drive transistor and the transistor T2 is an input transistor. Note that in the example shown in FIG. 37, the transistors T1 and T2 are n-channel thin-film transistors (TFTs).
The transistor T1 is provided in series with the organic EL element OLED. The transistor T1 is connected at its gate terminal to a drain terminal of the transistor T2, connected at its drain terminal to a power supply line that supplies a high-level power supply voltage ELVDD (hereinafter, referred to as “high-level power supply line” and denoted by the same reference character ELVDD as the high-level power supply voltage), and connected at its source terminal to an anode terminal of the organic EL element OLED. The transistor T2 is provided between the data line DL and the gate terminal of the transistor T1. The transistor T2 is connected at its gate terminal to the scanning signal line SL, connected at its drain terminal to the gate terminal of the transistor T1, and connected at its source terminal to the data line DL. The capacitor Cst is connected at its one end to the gate terminal of the transistor T1 and connected at its other end to the source terminal of the transistor T1. A cathode terminal of the organic EL element OLED is connected to a power supply line that supplies a low-level power supply voltage ELVSS (hereinafter, referred to as “low-level power supply line” and denoted by the same reference character ELVSS as the low-level power supply voltage). A connecting point among the gate terminal of the transistor T1, the one end of the capacitor Cst, and the drain terminal of the transistor T2 is hereinafter referred to as “gate node” for convenience sake. A gate-node potential is denoted by reference character VG. Note that although in general, one of the drain and source that has a higher potential is called a drain, in the description of this specification, one is defined as a drain and the other is defined as a source, and thus, a source potential may be higher than a drain potential in some cases.
FIG. 38 is a timing chart for describing the operation of the pixel circuit 91 shown in FIG. 37. Prior to time t91, the scanning signal line SL is in a non-selected state. Therefore, prior to time t91, the transistor T2 is in an off state, and the gate node potential VG keeps its initial level (e.g., a level determined according to writing performed in the preceding frame). At time t91, the scanning signal line SL goes into a selected state and thus the transistor T2 is turned on. By this, a data voltage Vdata corresponding to the luminance of a pixel (subpixel) formed by the pixel circuit 91 is supplied to the gate node through the data line DL and the transistor T2. Thereafter, during a period until time t92, the gate node potential VG changes according to the data voltage Vdata. At this time, the capacitor Cst is charged to a gate-source voltage Vgs which is the difference between the gate node potential VG and the source potential of the transistor T1. At time t92, the scanning signal line SL goes into a non-selected state. By this, the transistor T2 is turned off, and the gate-source voltage Vgs held in the capacitor Cst is fixed. The transistor T1 supplies a drive current to the organic EL element OLED, according to the gate-source voltage Vgs held in the capacitor Cst. As a result, the organic EL element OLED emits light at a luminance according to the drive current.
Meanwhile, the pixel circuit 91 shown in FIG. 37 is a circuit corresponding to one subpixel. Therefore, a configuration of a pixel circuit 910 corresponding to one pixel including three subpixels is as shown in FIG. 39. As shown in FIG. 39, the pixel circuit 910 forming one pixel is composed of a pixel circuit 91(R) for an R subpixel, a pixel circuit 91(G) for a G subpixel, and a pixel circuit 91(B) for a B subpixel. According to the configuration shown in FIG. 39, since many circuit elements are required within a pixel circuit, it is difficult to achieve definition improvement.
In view of this, Japanese Patent Application Laid-Open No. 2005-148749 discloses, as shown in FIG. 40, a pixel circuit 920 configured to further reduce the numbers of transistors and capacitors that are required for one pixel over the conventional one. The pixel circuit 920 is composed of a driving means 921, a sequential control means 922, and three organic EL elements OLED(R), OLED(G), and OLED(B). The driving means 921 is composed of a drive transistor T11, an input transistor T12, and a capacitor Cst1. The sequential control means 922 is composed of a transistor T13(R) for controlling the light emission of the red-color organic EL element OLED(R), a transistor T13(G) for controlling the light emission of the green-color organic EL element OLED(G), and a transistor T13(B) for controlling the light emission of the blue-color organic EL element OLED(B). In addition, as wiring lines for controlling the on/off of the transistors T13(R), T13(G), and T13(B), emission lines EM1, EM2, and EM3 are provided so as to pass through the pixel circuit 920.
In a configuration such as that described above, one frame period is divided into three subframes. Specifically, one frame period is divided into a first subframe for performing red light emission, a second subframe for performing green light emission, and a third subframe for performing blue light emission. Then, in the sequential control means 922, only the transistor T13(R) is brought into an on state in the first subframe, only the transistor T13(G) is brought into an on state in the second subframe, and only the transistor T13(B) is brought into an on state in the third subframe. By this, the organic EL element OLED(R), the organic EL element OLED(G), and the organic EL element OLED(B) sequentially emit light over one frame period, displaying a desired color image. In the organic EL display device disclosed in Japanese Patent Application Laid-Open No. 2005-148749, the numbers of transistors and capacitors required for one pixel are reduced in the above-described manner. Note that Japanese Patent Application Laid-Open No. 2005-148750 also discloses a pixel circuit configured to be provided with a plurality of transistors for controlling the light emission of organic EL elements for respective colors, and provided with a plurality of emission lines for controlling the on/off of the plurality of transistors.